A broad range of ultrasonic transmission and receiving systems are deployed in salt water by means of electric cables. In order to maintain the electrical insulation of the cable connections, the various existing cable junctions must be encapsulated in a resin system which has good electrical properties, a high order of adhesion to the various cable sheaths and metallic components and is immune to the deteriorating effects of sea water. Ideally, such encapsulating system should be capable of being applied under adverse field conditions by unskilled personnel.
The encapsulant which is presently used to make there terminations and also cable splices is an epoxy resin crosslinked with a polyamide-imide sold under the trademark "Versamid." This encapsulating system is quite viscous and entraps large quantities of air during the catalyst-base resin-mixing process. Unless the air is removed by means of a vacuum-degassing process, the encapsulated connectors will contain large voids and porous areas which render them useless for the intended application. Degassing is done by placing a container of the catalyzed mixed resin system in a vacuum desiccator and holding the resin at reduced pressures until it no longer emits gas bubbles. Great care must be exercised in the degassing operation because the optimum resin temperature for degassing is 50.degree. C. At lower temperatures, the resin system is so viscous that it cannot be deaerated satisfactorily, and at a temperature of 60.degree. C. the system has a working life of only 5 to 10 minutes. Even a properly degassed system has such a high viscosity that great care must be exercised in filling the space in order to prevent the inclusion of voids caused by fold-overs in the poured resin stream. The high degree of precision needed to successfully use this system makes it difficult to use it in the field.
In spite of these problems, the present epoxy resin system does have the advantages of excellent adhesion to the various system components, a range of flexibilities, transparency, and the capacity to cure in 24 hours at room temperature. The ability of these resins to cure at temperatures below 80.degree. C. is a critically important property because the delicate electronic devices which are encapsulated by the resins can be damaged at temperatures over 80.degree. C.